Methods for the authentication of objects such as, for example, documents or banknotes are known in the prior art.
U.S. Pat. No. 4,218,674 discloses a system and a method for checking the authenticity of a document, wherein binary output signals generated on the basis of the document are compared with previously stored binary signals. The document contains a security marker in the form of randomly distributed fibers composed of a magnetic or magnetizable material. For the purpose of reading the security marker, the document is scanned along a predetermined track by means of a detector, which registers magnetic fields and outputs an electrical pulse upon crossing the magnetic or magnetized fibers.
DE 103 04 805 A1 describes a method for producing security markers, which utilizes a random pattern which is present on an object to be marked or is applied thereto. For this purpose, the random pattern is read into a computer by means of a reader and a fingerprint is extracted, which includes individual features of the pattern. Optionally, an identification number is applied on the object. The extracted fingerprint is stored in a mechanical data storage device. In order to identify the marked objects, the random pattern from the object is read in, the fingerprint is extracted and compared with the fingerprint stored in the data storage device.
DE 60 2004 007 850 T2 discloses a method, a computer program and an electronic device for determining the authenticity of an object, wherein the object has a three-dimensional pattern of randomly distributed particles. The method employs a first and second code. The second code is determined by two-dimensional data acquisition on the pattern of randomly distributed particles. For this purpose, the object is illuminated with white scattered light and the light reflected and transmitted by the object is detected. The object, comprising a pattern of randomly distributed particles, is preferably a label.
The security markers known in the prior art can be assigned to two groups:    (a) The security marker is an inherent constituent part of the product which arises randomly during production or is produced by means of targeted measures. In this case, narrow limits are imposed on the type and constitution of the marking on account of the material composition, surface structure and form of the product. Known product-inherent markers include, inter alia, optically detectable random surface patterns formed from scratches or fibers or precisely defined isotope admixtures in polymeric materials. Product-inherent security markers have a narrowly restricted field of use and are unsuitable for foodstuffs, medicaments, cosmetics and clothing textiles.    (b) The security marker is configured as a label and is fitted on the product. Labels have the disadvantage that they have a restricted area and facilitate the localization and identification of the security marker. By means of modern, commercially available instruments appertaining to metrology and analysis, the physicochemical constitution and the functional principle of the security marker can generally be determined rapidly. If the constitution and the functional principle are known, at best a copy protection stands in the way of counterfeiting. The prior art describes two methods for forming a copy protection, the two methods also being combined. Firstly, an “invisible” security marker is proposed and, secondly, a non-reproducible security marker or security marker that is reproducible only with a disproportionately high outlay is proposed.
With regard to the copy protection of security markers, the following aspects play an important part:
i) Reproducibility
A security marker should as far as possible not be reproducible. In this case, the term “reproducible” should not be understood in the sense of an exact physical replication, but rather with regard to the metrological detection of specific patterns present in the security marker. Known security markers usually employ spatial—generally two-dimensional patterns such as smart codes, for example, which are detected by means of optical or magnetic detectors. Primarily holograms can be mentioned as an example of three-dimensional patterns. Less customary security markers comprise chemical markers such as isotopes, for example, which are detected by means of spectrometric measurement methods.
In order to reproduce a security marker, the pattern firstly has to be identified. The identification of a pattern can be made more difficult in various ways, inter alia by using a pattern which is not visible to the human eye. Hidden (so-called covert) patterns are thus proposed in the prior art. However, most of the known invisible patterns can be identified with little outlay by means of measurement methods available nowadays.
After identification it is necessary to recreate or reproduce the pattern in such a way that the reproduction is indistinguishable from the original during metrological detection. In principle, any identified pattern can be reproduced, but the outlay required for this purpose is accorded crucial importance. If the outlay on reproduction exceeds the economic advantage resulting therefrom, then the reproduction is not worthwhile and fails to occur. The outlay on reproduction is closely related to the metrological detection of the pattern. The simpler the metrological detection is made, the lower the outlay generally required on reproduction.
The information content of security markers is furthermore important. In this case, the term information content should be understood as a synonym for the number of structure details, such as points or lines, for instance. The higher the information content, the more outlay required for replication. The information content is upwardly limited by the area ratio of the security marker with respect to the size of the detail structures. The larger the area of the security marker and the smaller the detail structures, the greater the maximum possible information content.
ii) Metrological Detection
The metrological detection of security markers is generally effected at two or more locations and/or points in time, e.g. by the manufacturer of a product, if appropriate in a freight warehouse or during transport and by a trader or in the course of sales. In this case, a product is firstly equipped with a security marker in a marking step. The security marker or the pattern contained therein is generally not known a priori, but rather is detected metrologically and the measurement signal is recorded in encrypted or unencrypted form as an identity code. In a later identification step, a security marker situated on a product is detected metrologically in a manner similar to that in the marking step and the measurement signal in encrypted or unencrypted form is compared with existing identity codes.
During metrological detection, the product provided with a security marker is positioned under a detector or guided past a detector. This last is the case, for example, with laser scanners, magnetic read heads or cameras with a line sensor, such as are customary in industrial image processing. The positioning or movement of the product relative to the detector is effected manually or by means of a mechanical device such as a conveyor belt, for example. In this case, it is necessary to comply with specific stipulations on account of production-technological or logistical conditions. It is often necessary or desirable for the metrological detection to be effected in a contactless manner, wherein the working distance between the product and a detector must not fall below a minimum distance of from a few cm to a few meters. If the working distance is intended to be more than a few cm, preferably optical, in particular imaging, methods are used for the metrological detection. In this case, important measurement parameters such as resolution, image field and working distance cannot be set arbitrarily but rather influence one another in accordance with the laws of optics. In addition, albeit to a smaller extent, the choice of the measurement parameters is restricted by the camera lens used. With the camera lenses designed for industrial demands, in contrast to high-performance lenses for astronomical or satellite engineering applications, the possibilities of optical metrology cannot be fully exhausted.
The metrological detection of security markers has to satisfy different, in part contradictory, requirements; these include:                high sensitivity, such that slight deviations of a copied security marker from the original are recognized. In the case of optical detection of two-dimensional patterns, sensitivity means primarily high lateral resolution and contrast, that is to say that the optical measurement system used has to have an optimized modulation transfer function.        Immunity to metrological deviations in order that the false-positive error rate, that is to say the number of original security markers falsely recognized as reproduction, is low. A frequent metrological deviations during optical detection is incorrect positioning of the security marker relative to the detector, vibrations or different lighting conditions.        Low costs for procurement and operation of the measurement system.        High speed or high throughput.        Automation.iii) Coding        
In the present case, the term coding subsumes all known electronic and mathematical methods used in the metrological detection, conversion, encryption, storage and reproduction of security markers. These methods can be implemented in the form of electronic hardware or software. The volume of data used during coding is substantially determined by the information content of the security marker in conjunction with the resolution capability of the metrological detection. During the optical detection of two-dimensional patterns, the volume of data is upwardly limited by the product of the number of metrologically resolved pixels (resolution pixels) and the number of color or contrast levels per resolution pixel. Detailed structures of the security marker which are smaller than the resolution pixel cannot be detected and hence cannot be coded.
A central problem of the optical detection of two-dimensional security markers is briefly explained below. For the ratio of lateral resolution and depth of focus of a camera lens, the following relationship holds true to a good approximation:Δz=Δx2/λwhere Δz denotes the depth of focus, λ denotes the wavelength of the light used for the imaging, and Δx denotes the lateral resolution on the object side. The center of the visible light spectrum is at approximately 500 nm. If this value is inserted into the above equation, i.e. λ=500 nm, this results in the following:
Δx1510152030405060[μm]Δz0.0020.050.20.50.81.83.25.07.2[μm]The depth of focus Δz indicates the depth range in which two lines or points lying alongside one another at a distance Δx on the security marker are still imaged as separate objects in the image plane. In order that a structure detail having a size of 40 μm is still imaged sharply, it must not be at a distance of more than 3.2 mm from the focal plane of the camera lens. Accordingly, the optical detection of security markers with a resolution of ≦40 μm requires precise positioning of the security marker relative to the detector with a tolerance of ≦3.2 mm. A small lateral offset or a small angular difference of the security marker during optical detection in the identification step relative to the marking step would bring about a false-positive error recognition. In order to avoid this problem during high-resolution optical detection, it is necessary to use high-precision mechanical positioning systems alongside a suitable camera lens. If this is compatible with logistical conditions, the use of precise mechanical positioning or transport systems is often prohibited for cost reasons.
It should additionally be taken into consideration that structure details having a size of around 40 μm are simple to reproduce by means of the technical possibilities available nowadays. This is because commercially available laser or inkjet printers for the private user already have resolutions of 600 to 2400 dpi (42 to 11 μm). Such printers are thus suitable for reproducing two-dimensional security markers with structure details in the range of 10 to 40 μm.
It is evident from the above explanations that security markers with very fine structure details of 40 μm are unsuitable for numerous commercial applications.